1. Field of the Invention
This invention relates to data communications over a general switched telephone network, and more particularly to a method and apparatus for determining whether a sufficiently clear path exists through a general switched telephone network for digital communication between a first and a second endpoint on a telephone system.
2. Description of Related Art
Telecommunications is currently undergoing a dramatic change due to the transition from analog technology to digital technology in the general switched telephone network (GSTN). Digital telephone service, such as integrated services digital network (ISDN) service, is becoming available in a number of regions or "islands." As digital telephone service becomes available to more telephone service subscribers, a number of advantages and challenges are generated.
In a traditional analog link, information is transmitted by a subscriber's equipment (typically a telephone or modem) as an analog signal. Presently, for an interoffice call, it is typical for the analog signal to be routed to a central office, where the analog signal is converted to a digital signal by a coder/decoder (CODEC) device. A CODEC is essentially an analog-to-digital/digital-to-analog converter which samples the analog signal at a predetermined rate in a first direction, and receives and decodes a digital data stream of samples in a second direction. In the first direction, the output of the CODEC is a stream of digital words ("samples") that are each representative of the weighted amplitude of the received analog signal at particular sample times. The samples output by the CODEC can then be switched by digital switches and routed to a second central office to which a second party to the call is connected. In the second direction, the output of the CODEC is the analog signal represented by the received stream of samples. A CODEC at the second central office operates in a similar manner to encode an analog signal received from a second subscriber and decode the stream of samples received from the CODEC at the first central office.
However, it is currently becoming common for information to be placed onto the telephone system in digital form. That is, the subscriber's equipment outputs "end user digital information" that is accepted by the telephone system. The end user digital information, having already been converted to digital form in the subscriber's equipment, does not pass through a CODEC. Rather the end user digital information is directly switched by the digital switching equipment at the central office. A digital signal including the end user digital information is then routed to the central office associated with the party receiving the call. That central office then extracts the end user digital information from the received digital signal and switches the end user digital information directly to the receiving party. In the context of the present invention, the term "samples" refers to end user digital information as well as the output of devices such as CODECs, even though such end user digital information is not necessarily a "sample" of an analog signal.
One advantage that comes from the subscriber directly placing end user digital information on the telephone system is that digital information can be transmitted between the subscribers directly in digital form. Therefore, there is no need to modulate information that is already in digital form onto an analog signal. A further advantage is that it is possible to communicate at higher data transfer rates with much lower bit error rates, since digital signals are less susceptible to interference due to environmental noise and limited bandwidth than are analog signals (such as signals output by modems). Modems that are used with POTS are limited by practical considerations to data transfer rates of less than 30 kbps. Therefore, the amount of information per unit time that can pass through an analog telephone connection is limited.
However, because the current telephone infrastructure is an elaborate and extensive network, a transition from analog services to all digital service will take several years. Therefore, as the transition is being made, there will be areas in which digital service is available to a subscriber, and areas in which only analog service will be available to a subscriber. Furthermore, the routing of a call through the GSTN is complex and may include a number of legs. Each such leg may include analog equipment. Each call may be routed differently at different times, even if made by the same first subscriber to the same second subscriber. This means that a subscriber who wishes to originate a call to another subscriber for the purpose of transmitting and receiving end user digital information must know the capabilities of the telephone equipment between the two subscribers involved in that particular call before that subscriber can determine the most appropriate data transfer method and rate for the transmission. There is currently no way for a subscriber to determine whether analog or digital equipment is being used along the path of a call that is made through the GSTN.
FIG. 1 is an illustration of five subscribers of telephone service. Currently, end user digital information can be transmitted from one subscriber to another over circuit switched facilities using one of three methods. The first method requires each subscriber to have a modem. The modem modulates end user digital information using an analog modulation scheme (such as V.22bis, V.32, etc.). The received signal is demodulated by an inverse operation. A call can be placed and data transmitted by any subscriber capable of placing a voice call. Likewise, the call can be received and the end user digital information decoded by any subscriber capable of receiving a voice call, if equipped with the appropriate modem. The second method requires that end user digital information be transmitted over switched 56 kbps facilities. End user digital information is sent in digital form over the switched 56 kbps facility to other subscribers using the same service. The third method requires each subscriber to be connected to a Basic Rate Interface (BRI) or Primary Rate Interface (PRI) in the same ISDN "island" (BRIs and PRIs are well established interfaces within the telecommunications industry). An ISDN island refers a collection of subscribers that are capable of making end-to-end ISDN data bearing calls to one another, even if served by a different switch.
In FIG. 1, subscriber A is provided with a BRI 1 from an end office 2. In a BRI, two B-channels and one D-channel are available to the subscriber. The two B-channels each have a native data rate of up to 64 kbps. Each B-channel can transmit either voice or data information. The D-channel has a data rate of up to 16 kbps. The D-channel is typically used for signaling (e.g., call set-up, tear-down, progress indication, etc.), but may be used for transmitting data. Since a BRI has a separate channel for signaling, the entire 64 kbps bandwidth of each B-channel can be used for data.
Subscriber B is also provided with a BRI 4 from a different end office 6. The end offices 2, 6 are within the same ISDN island 8. Therefore, subscriber A can call subscriber B through a digital path and take full advantage of the 64 kbps data transfer rate that is possible between subscribers BRI within the same ISDN.
Subscriber C is provided with a BRI. However, subscriber C is connected to an ISDN island 12 that is not part of the ISDN island 8 to which subscriber A and subscriber B are a part. An ISDN 64 kbps data call between subscriber C and subscriber A or B will fail since subscriber C and subscribers A and B are part of different islands. A second call must then be placed requesting voice capabilities. This is because an ISDN island will not permit a call requested with data bearing capabilities to be made when the path must use GSTN equipment not under the island's control. The ISDN will not allow such a call because within the GSTN 13 the path may be routed through analog equipment, or the digital data transmitted through the GSTN 13 may be altered by signaling arrangements sharing the call path (such as the commonly known "robbed-bit signaling" technique). However, since subscribers A, B and C are each connected to the GSTN 13 through the respective ISDN islands 8, 12, data can be transmitted between subscriber A or B and subscriber C by using a modem.
As shown in FIG. 1, subscriber D can exchange digital data using switched-56 kbps facilities delivered over T1, but then only to specific subscribers of that service. Subscriber D can also exchange digital data with any other subscriber, if an analog modulation scheme is used (i.e., modems are used), but only at a comparatively limited information rate. A digital data call can only be established between subscriber D and other subscribers to the same digital facility.
Subscriber E of FIG. 1 is connected to the GSTN 13 by a POTS line 16. Therefore, subscriber E is limited to transmitting and receiving only analog signals. Analog signals are suitable for voice conversation or modems.
If subscriber A wishes to transfer data between itself and subscriber E, such a transfer would be possible by encoding the information in accordance with a standard analog modulation technique. This requires special circuitry at the subscriber A end of the line, since subscriber A is interfacing with the GSTN 13 through the ISDN island 10 directly in a digital format. Subscriber A encodes the data initially placed on the BRI 1 in the same manner as it would be encoded were the data to have been sent from a standard modem and encoded in a CODEC at the central office. Thus, when the data is decoded at the CODEC at the central office connected to the subscriber E and demodulated by the modem at subscriber E's site, the information will be correct. The equipment at subscriber A must also have knowledge of the capabilities of subscriber E before placing the call. Typically, subscriber A would request a call with 64 kbps bearer channel capabilities. This call would fail, and subscriber A would then have to place a second call requesting voice capabilities. This second call would typically succeed.
As described above, the current state of the art has the following problems:
1. Current state of the art analog modulation schemes are limited to a data exchange rate of 28.8 kbps. Operation above 30 kbps appears to be impracticable at any time in the future. PA1 2. Using switched digital 56 kbps facilities does not allow flexible connection to other subscribers not using the same facility. For example, a switched 56 kbps subscriber cannot interoperate with a subscriber limited to analog modulation schemes. PA1 3. BRIs alone are not sufficient to permit exchange of digital data over the plurality of interfaces available today. As discussed above, BRI subscriber can make digital calls only to BRI subscribers of the same ISDN island. PA1 4. BRIs can interoperate with subscribers limited to analog modulation schemes only if the capabilities of the end subscriber are known before placing a call. PA1 5. ISDNs can provide exchange of digital data only between subscribers of a particular ISDN island. Subscribers in a different ISDN island are prevented from capitalizing on existing digital facilities.
The present invention provides an apparatus and method for allowing a first subscriber to automatically determine whether digital operation is supported by a link to a second subscriber after establishing the link, and the highest data rate supported by the link. The present invention also provides a method and apparatus for allowing a subscriber on a BRI to originate 56 kbps or 64 kbps digital calls to, and receive calls from, other subscribers having such digital capability. Still further, the present invention allows a subscriber on an ISDN to originate a call to a subscriber who is capable of communication only by analog modulation schemes without placing a first call that will fail and then having to make a second call, and without knowing the capabilities of the subscriber prior to placing the call.